Conventionally, a clutch hub in a dry type single plate clutch has been manufactured by punching a steel sheet obtained by annealing a hot-rolled steel sheet of an alloy steel such as SCM435, SCr435 or SCM440 as a raw material into a predetermined configuration, and subjecting the punched steel sheet to heat treatment. These alloy steels, however, exhibit high hardness as the raw material so that there may occur forming defects such as distortions, tears, etc. in the punching step.
Accordingly, it can be considered to accelerate the softening of the raw material by adjusting the annealing temperature or extending the annealing time. In this case, cost for the raw material increases, but stable punchability inclusive of stable tool life has not been sufficiently effected.
In addition, the above-described alloy steels exhibit the problem that toughness after heat treatment is low. In order to improve toughness, the tempering temperature may be raised, but, in this case, there occurs lowering of strength so that both desired strength and desired toughness cannot be effected.
Furthermore, there also occur distortions in the alloy steels due to punching and following heat treatment so that the correcting step of correcting distortions, such as press-tempering, must be provided after heat treatment. Consequently, there are also exhibited the problems that the productivity is lowered, and the manufacturing cost is increased.
Publication of unexamined Japanese patent application No. Hei5-098357 discloses the technique relating to the production of a high carbon steel sheet excellent in formability and toughness. However, in the steel composition disclosed therein, the Mn content is as high as 0.6 to 1.50 mass % so that even if the softening step is carried out by annealing prior to forming, sufficient improvement in punchability cannot be expected.
And Publication of unexamined Japanese patent application No. 2002-121647 discloses a heat treated steel sheet excellent in formability and abrasion resistance, and its production method. And this publication proposes the structure containing spheroidized cementite and graphite with an area rate of 10% or less and with a diameter of 10 μm or less as the internal structure of the steel sheet prior to working. However, high frequency induction heating is exemplified as the heat treatment, and this technique cannot be directly applied to the manufacturing of the clutch member.